1. Field of the Invention
The present invention relates to a process for the production of a magnetic recording member. More particularly, the present invention is concerned with an improved method of coating magnetic compositions.
2. Description of the Prior Art
In general, a magnetic recording member is produced by providing a magnetic layer on a flexible or hard non-magnetic support. For example, in producing a magnetic tape, etc., a magnetic coating composition containing a mixture of a magnetic powder and a binder is coated on the support in layer form to provide the magnetic layer. The magnetic layer is usually in mono-layer form, not in multi-layer form.
However, in order to produce magnetic tapes particularly suitable for recording signals of short wave length and high performance tapes in which uniform output is required over a wide frequency range, magnetic recording members carrying magnetic layers of various multi-layer constructions have been proposed.
U.S. Pat. Nos. 2,691,072, 2,643,130, 2,941,901, Japanese Patent Publications 2218/1962, 5351/1965, U.S. Pat. No. 3,761,311, Japanese Patent Laid Open No. 31907/1973, etc., describe such multi-layer constructions for obtaining uniform output.
U.S. Pat. No. 2,647,954 describes a magnetic recording member in which coercive force is continuously changed in the thickness direction for the purpose of obtaining the same effect as above.
Japanese Patent Publication 8106/1964, U.S. Pat. No. 3,416,949, etc., describe methods of producing magnetic layers free from defects by providing them as a multi-layer construction.
In Japanese Patent Publication 23678/1964 and Japanese Patent Laid Open 18508/1972, the mechanical properties of tapes are taken into account. In U.S. Pat. Nos. 3,676,217, 2,941,901, Japanese Patents Laid Open 81093/1973, 37903/1972, etc., head abrasion characteristics and contact conditions between recording heads and a tape are taken into account.
Japanese Patent Publication 185/1968, U.S. Pat. No. 3,328,195, etc., disclose the use of such multi-layer recording members for multiple recordings.
Japanese Patent Publication 28681/1973, Japanese Utility Model Publication 19135/1959, etc., describe that properties such as the S/N ratio, etc., are improved by employing multi-layer constructions.
Japanese Patent Laid Open 98803/1973 discloses a method of simultaneously providing a plurality of layers, that is, in accordance with this method multi-layers are simultaneously coated so that modulation noise due to any unevenness between two layers, which is liable to be formed by providing the layers separately (i.e., after the lower layer is coated and dried, the upper layer is coated) is reduced.
As a result of the inventors' investigations on these methods, it has now been found that even if complicated processes of placing one magnetic layer on top of another are employed, good effects are not necessarily obtained. In particular, in the method of separately providing two layers, the surface properties of the upper layer (upper magnetic layer) are liable to deteriorate, and a decrease in magnetic properties, particularly the squareness ratio (Br/Bm) of the upper layer, and an increase in noise are observed. These undesirable phenomena are especially shown when the lower magnetic layer is thicker than the upper magnetic layer.
The method of simultaneously providing the layers somewhat reduces modulation noise due to unevenness formed between the layers. With this method, however, problems still occur in that coating heads are complicated, it is impossible to change the direction of the orientation of the magnetic substances in the upper layer and the lower layer (lower magnetic layer), and it is desired that the magnetic coating composition for the upper layer and the magnetic coating composition for the lower layer have comparatively similar properties, etc.